Achieving both high current density and high selectivity for high-value products is crucial for the widespread implementation of CO electrolysis. Toward high-current-density electrolysis, it is crucial to design a triple-phase interface, where the catalyst, electrolyte, and gaseous substrate intersect, serving as the active reaction site for CO electrolysis. In this study, aims to establish design principles for the triple-phase interface composed of copper nanoparticles (CuNPs) to achieve ultra-high-current-density electrolysis of gaseous CO into multicarbon (C) products. The C formation activity of electrodes carrying various CuNPs is systematically evaluated under high-current-density (>1 A cm) electrolysis conditions. By analyzing the correlations between the electrochemical performances and the physicochemical properties of the catalysts and electrodes, it is identified that the average size of interparticle spacing in the catalyst layer is correlated with the maximum partial current density for C production (j). Smaller interparticle spacings are found to enhance j by suppressing the excessive electrolyte penetration into the catalyst layer and forming an expansive triple-phase interface. Based on these insights, the optimized electrode, with an average interparticle spacing of 59.4 nm, exhibited a record j of 2.00 A cm with a faradaic efficiency of 80.1%.